Ceramic membranes prepared from oxide ceramics have been known for several years. In comparison to polymeric membranes, ceramic membranes have much higher temperature capabilities (e.g. alumina can be used up to about 800.degree. C.). They are also highly inert chemically. They can be repeatedly sterilized at 120.degree. C. or higher. Since the creation of the first ceramic membranes, the interest in using them at more and more severe conditions, i.e. at higher operating temperatures and at more extreme pH's, has increased. However, the inherent physical limitations of the oxide ceramics together with the difficulty in reproducibly manufacturing ceramic membranes has limited their utility. The present invention produces ceramic membranes which are generally more stable at extreme conditions than previous ceramic membranes. This invention also enables the manufacture of ceramic membranes to be performed quite reproducibly.
U.S. Pat. No. 4,698,157 discloses supported filter membranes prepared from silicon nitride particles which are then sintered to fixedly interconnect them. As in any process relying solely upon heating for an extended time to cause a reproducible physical action, there is a great degree of non-reproducibity inherently created. Thus, slight variations in the heating temperature and/or time can cause wide variation in the pore sizes produced in the final product.
U.K. Pat. No. 1,490,543 discloses preparing unsupported porous silicon nitride materials by the nitridation of a composition of a silicon powder, a binder powder, and an inert removable additive powder. The porosity of the material is stated to be generated by the removal of the additive powder. The resultant bodies have total porosities of 50% or more. The fine pores required of a microfilter or an ultrafilter are not obtainable by this procedure. Also the thicknesses of the resultant bodies would create excessive pressure drops across them during actual use thereby effectively precluding use in the fine separations to which the present invention is directed.